During operation, an industrial robot has to meet high requirements regarding precision and accuracy. The industrial robot carries out defined reproducible movements over a program-controlled linkage system driven by means of electric motors. The dimensions of the movement of the individual driving groups is controlled and supervised by an electronic path-measurement system. The zero positions of the driving system and the path-measurement system must correspond and be set to correspond again should deviations occur. An industrial robot thus has a need for a device for the exact and reproducible setting of the motor-driven linkage system of the robot. It is necessary to calibrate a robot to fulfil the demands for precision and accuracy named above and, following periods of stoppage of production and after service, it is necessary to synchronise the robot in order to meet the demands named above.
Here, calibration refers to initiating a configuration of a robot at start-up. The aim is to find the exact configuration (zero-position/calibration position) of the robot at the time when turning on the power and initiate the verifying part of the control system accordingly.
Here, synchronisation refers to checking or setting the robot to the zero-position/calibration position following service stops (e.g. replacing a motor), stoppages of production, collision and the like.
Industrial robots working with high accuracy are equipped with servo-controlled motors. To check a motor, a sensor has to measure the exact position of the angle of the motor axle in real time. In servo motors, positional information together with angular speed is used in a feedback system to clear differences between predetermined and actual positions. Positional information cannot be used to check the robot if the measurements do not reflect the actual position of the robot. The goal of calibration is thus to initiate measurements of its “true/real” values.
When working with industrial robots, the need thus arises for a method for positional measuring to be used when calibrating and synchronising a robot.
The American document U.S. Pat. No. 4,419,831 specifies a device that allows calibration of a linkage system that can be brought to correspond accurately with the calibration of an electronic path-measuring system even after replacement of parts of the drive or path-measuring system respectively. In a moveable part of two associated linkage parts, a recess or an elevation is arranged, and in an associated second linkage part, a guide for a measuring sensor instrument is adjustably and fixably arranged, where the sensing element of the instrument indicates the calibration position/zero-position during relative movement of both the linkage parts by determining the minimum respectively maximum of the recess or elevation. The object of the measurement method is to get the possibility to use the same program following replacement of measuring system or manipulator.
Service stops and other shut-downs mean disturbance of production lines and non-production, which leads to large and unwanted costs. It is thus of greatest importance that the time for shut-downs is minimised. Since the robot must be calibrated/synchronised after every stoppage, it is important that this is carried out quickly. At the same time, it is of greatest importance that the method is simple, accurate and has good reproducibility. Thus, the need arises for an accurate, simple and quick method for positional measuring.
This need cannot be met by the method in the American patent.